The present invention relates to a method of preparing polymers in an aqueous medium. More particularly the invention relates to the preparation of polymers in aqueous medium by the free radical addition polymerization of monomers in an aqueous medium in the presence of a non-salt forming free radical polymerization initiator and a salt or partial salt of an acid-containing polymer having one or more pendent double bonds.
U.S. Pat. No. 3,258,437 discloses the preparation of polymers of butadiene in aqueous medium by a method which involves addition polymerizing diene or diene plus an ethylenically unsaturated monomer in water in the presence of a water-soluble free radical polymerization initiator and an amine or ammonia solubilized salt of an adduct of a drying oil of a long oil alkyd and a dicarboxylic acid or anhydride. This method while advantageous in many ways nevertheless has some limitations. For example, the stability of the oil adduct salt and hence the overall stability of the resultant polymer emulsion is not as good as one would desire. Moreover, the process is not readily adaptable to the preparation of other aqueous polymer emulsions. Finally, the products produced by the process while excellent for many coating applications and methods are not suitable in certain coating applications such as electro deposition due to the presence of the water-soluble polymerization catalyst employed.
It is also known to prepare an amide-containing acrylic interpolymer in aqueous medium by a process involving the free radical addition polymerization of the monomers in aqueous medium in the presence of an oil-soluble free radical polymerization initiator and an amine salt of an acrylic interpolymer of an unsaturated carboxylic acid amide, an unsaturated carboxylic acid and other ethylenically unsaturated monomers. This method is advantageous in that it produces polymers having excellent properties. However, this method also has its disadvantages. Thus, with certain polymers the method does not always produce polymer emulsions having the desired degree of stability and compatibility.